Oblique foliation
Ingeologyoblique foliation,steady state foliationoroblique fabricis a special type of atectonicallyproducedfoliationorfabric,most commonly inquartz-rich layers. Themicrotectonicstructurecan be used to determine theshear senseinshear zonesand their associated rocks, usuallymylonites.
Description of the structure
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Oblique foliation is mainly encountered in shear zones, where it forms as a result of the shearingdeformationswithin the affected zone. As the name implies, this foliation/fabric takes on an oblique attitude to the shear-zone boundary (i.e. thefabric attractor) and internal layering, usually an angle of about 20°–40°, or less. (In some shear zones, even angles less than 5° are reported, yet angles steeper than 45° are also known). A closer look reveals that the foliation/fabric is created by the parallel arrangement of a multitude of similar oriented small grains, which are slightly elongated in the foliation direction.[1]Oblique foliation thus represents primarily a shape-preferred orientation (SPO).
In their geometrical arrangement, oblique foliations are somewhat similar to (type I)S-C-fabric,in which the elongated grain fabric becomes a trueschistosity/foliation. Occasionallymica fishget incorporated into oblique foliations; this structure has been calledtype II S-C-fabricby Lister & Snoke (1984).
Formation of the structure
[edit]Oblique foliation is a fabric that has achieved a steady state, but does not represent the total accumulatedstrain.
The structure is thought to result from the interplay of passive flattening and rotation of grains in a non-coaxial flow field on one hand andgrain boundary migrationdestroying the developing shape fabric at the same time on the other hand. The shearing deformation therefore is responsible for aligning the grains with the maximum extension direction of theincrementalstrain ellipsoid(theinstantaneous stretching axesor ISA), whereas the process of dynamicrecrystallisationopposes this by forming new equidimensional grains free of strain (by grain boundary migration); in order to achieve strain-free grains, a part of the developing shape fabric simultaneously must be destroyed.[2]
Hence, during progressive deformation, the foliation remains relatively stationary in orientation with respect to the kinematic frame of reference. Another consequence is that the orientations of an oblique foliation generally lag behind the attitude of thetotalstrain ellipsoid. The foliation never reaches the attitude of the flow plane and therefore only represents a part of the entire deformation history.
Occurrence
[edit]Oblique foliation has been found mainly in mono-mineral rocks, but can also occur in poly-mineral rocks. The structure occurs in the entiremetamorphic rangefrom low-grade to high-grade rocks. Major occurrences are mono-mineral layers of quartz,muscovite,andcalcitein layered mylonites. The structure has been described for quartz inquartzites,[3]for calcite incarbonates[4]and forolivineinperidotites.[5]Oblique foliation is also known to occur in rock analogues likeiceand syntheticoctachloropropane.
Theoretical considerations
[edit]The angle of the oblique foliation with the fabric attractor can be considered theoretically as a function of:
- the dynamicvorticity(number)Wk.
- thestrain ratedγ/dt.
- the recrystallisation rate (and therefore indirectly also the ambient temperature T).
By measuring the angle of the oblique foliation, attempts have been made to determineWk.Yet this method is problematic, because it neglects all the other parameters at work. Oblique foliations, whose angle with the fabric attractor exceeds 45°, pose a different problem difficult to reconcile with the available theory. One possible explanation for this seemingly paradoxical arrangement may be found intranstensionalshear zones transposing the ordinary oblique foliation into steeper attitudes by simultaneous extension.
Importance
[edit]Oblique foliations/fabrics find their most important use asshear sense indicatorsin mylonitic shear zones. The foliation/grain elongation always leans into the direction of shear, i.e. in a dextral shear zone, the foliation leans to the right and therefore dips to the left, and vice versa for sinestral shear. Combined with other shear sense indicators such asδ-objects,oblique foliations establish the movement sense quite clearly.
References
[edit]- ^Means WD. (1981). The concept of steady-state foliation.Tectonophysics,78:179–199.
- ^Ree JH. (1991). An experimental steady-state foliation.Journal of Structural Geology,13:1001–1011.
- ^Dell Angelo, LN & Tullis J. (1989). Fabric development in experimentally sheared quartzites.Tectonophysics,169:1–21
- ^De Bresser JHP. (1989). Calcite c-axis textures along the Gavarnie thrust zone, central Pyrenees.Geol. Mijnb.,68:367–376.
- ^Van der Wal D, Vissers RMD & Drury MR. (1992). Oblique fabrics in porphyroclastic Alpine peridotites: a shear sense indicator for upper mantle flow.Journal of Structural Geology,14:839–846.
Sources
[edit]- Passchier CW & Trouw RAJ. (1996).Microtectonics.Springer Verlag.ISBN3-540-58713-6
- Trouw RAJ, Passchier CW & Wiersma DJ. (2010).Atlas of Mylonites – and related Microstructures.Springer Verlag.
- Vernon RH. (2004).A practical guide to rock microstructure.Cambridge University Press.ISBN0-521-89133-7
